1. Field of the Invention
The invention relates to control over an idle rotation speed (hereinafter, simply referred to as ISC) in an internal combustion engine and, more particularly, to ISC in a cold state of a flexible fuel internal combustion engine that is able to use alcohol-containing fuel.
2. Description of Related Art
In an existing art, generally in an internal combustion engine for an automobile, or the like, ISC is executed. In ISC, an engine rotation speed during idle operation (idle rotation speed) is fed back, and an intake air amount and a fuel supply amount are controlled such that the idle rotation speed converges to a target rotation speed. The target rotation speed is basically set on the basis of an engine coolant temperature, a vehicle speed, and the like, and, when the engine is in a cold state or an air conditioner is operating, the target rotation speed is controlled to a relatively high value. Thus, an engine stop that is not intended by a driver (so-called engine stalling) is prevented (for example, see paragraphs 0047 and 0048 in Japanese Patent Application Publication No. 2010-14061 (JP 2010-14061 A)).
Incidentally, in recent years, not only gasoline or light oil but also fuel that contains alcohol (such as methanol and ethanol) may be used as fuel in an internal combustion engine of an automobile, or the like. This is described at paragraph 0015 in JP 2010-14061 A. A vehicle on which an internal combustion engine that uses alcohol-containing fuel (hereinafter, referred to as flexible fuel internal combustion engine) is mounted is generally called flexible fuel vehicle (hereinafter, simply referred to as FFV), and uses alcohol fuel to improve environmental performance, such as improvement in exhaust emission and reduction in the consumption of fossil fuel.
In the above-described flexible fuel internal combustion engine, various fuels having different blending ratios of alcohol and gasoline are usable, and 100 percent alcohol fuel is also usable. However, the physical property of alcohol differs from that of gasoline, so there is an appropriate way of control on the basis of the blending ratio. For example, ethanol has a lower volatility and is more hard to vaporize and has a smaller stoichiometric air-fuel ratio (A/F) than gasoline. Therefore, when ethanol is injected into an intake port, the influence of adhesion of fuel to a wall surface tends to be large.
Then, in the flexible fuel internal combustion engine, it is required to increase a correction amount corresponding to the amount of fuel adhering to the wall surface of the intake port in fuel injection control, and, particularly, the correction amount considerably increases in a cold state of the engine. Therefore, during idle operation in a cold state of the engine, the fuel injection amount may be excessive due to an increase in intake air amount through ISC. As a result, the air-fuel ratio becomes overrich, and a combustion state becomes instable, so hunting occurs in ISC, and vibrations and noise due to fluctuations in idle rotation speed may increase.
In addition, vaporization of alcohol having a low volatility mostly depends on a negative pressure in the intake port, so, when the alcohol concentration of fuel is high, although the fuel injection amount increases in response to an increase in intake air amount through ISC, fuel adhering to the wall surface is hard to vaporize due to a decrease in negative pressure (increase in pressure) in the intake port. This also presumably causes the above-described instability of combustion, and is one of factors of hunting in the control.
In contrast to this, when the alcohol concentration of fuel is high, it is conceivable to limit ISC in a cold state of the engine. However, by so doing, it is not possible to keep the idle rotation speed at a target rotation speed, and there is a concern that engine stalling occurs.